Immigration reform is a numbers game. And one of those numbers is $680—the price of applying for naturalization, the process that turns green card-holders into citizens. Politicians like Chicago Mayor Rahm Emanuel are decrying that number, saying the cost is too high. Non-profits, encouraging the country’s 8.5 million permanent residents to go down that path, are subsidizing that fee. So where does that total come from, and where does the money go?

The money goes into the coffers of the United States Citizenship and Immigration Services, a part of the Department of Homeland Security that oversees lawful immigration to the U.S. The total of $680 comes from a $595 application fee and a $85 “biometric fee” that is funneled toward background-check costs like fingerprinting. Applicants have to pay the $680 regardless of whether their application is successful—and sometimes it isn’t. In April, about 65,000 immigrants swore their oaths after being approved by the USCIS, while about 8,000 applications were denied.

The fee is both a practical and political issue. Practically, while many government agencies get cash through appropriations of taxpayer funds, the USCIS is primarily a “fee-based” agency. Today, a DHS official says, more than 90% of USCIS operating costs are covered by fees, so an immigrant paying the $680 can expect that moola to be spent on things like the salary of the employee adjudicating their case or the computer software that employee is using to crunch numbers. Fees like this also include a surcharge to help cover services that the USCIS provides for free, like naturalizing members of the military.

Politically, those who oppose fee hikes typically say that the cost discourages legal immigrants from becoming full-fledged citizens. In a 2012 Pew survey, Latino permanent residents who had not yet applied for naturalization were asked why. The top reason, cited by 26%, was difficulty with the English language (there’s a test) and other personal barriers; financial and administrative barriers came in third, at 18%. Those who support higher fees are likely to argue that American citizens shouldn’t be footing the bill for processing immigrants’ applications. The USCIS says they simply need that money to run the shop. “We’re not in the business of making a profit,” says USCIS Press Secretary Chris Bentley. “We’re certainly not in the business of losing money either. We’re in the business of breaking even.”

The USCIS is supposed to do a fee study every two years and propose fee levels proportional to the costs they’re incurring; those fees then get set through federal rulemaking. In 1997, the fee for a naturalization application was $95, less than 15% of what it is today. The most recent jump came in 2007, when the USCIS proposed increasing the naturalization application fee about 85%, from $320 to the current rate. “We must be able to recover the costs necessary to administer an efficient and secure immigration system,” then-USCIS Director Emilio Gonzalez said at the time. One promise the USCIS made in 2007 was that with additional money, the average time it takes to process an application would drop from seven months to five months; today, the average turnaround time is just under six.

Groups like the New Americans Campaign are pushing for naturalization, arguing that it’s good for the people—as citizens, they are more likely to have higher incomes and become homeowners—and the economy, which benefits from the extra money those immigrants will spend. Though the NAC doesn’t pay fees, they do donate legal counsel so that poor green card-holders can apply for waivers. Eric Cohen, who helps run the campaign, says those waivers are easy to get if the applicant is clearly in poverty. “We realize that when fees stay the same, when they go up … that has an impact on people’s lives,” USCIS’s Bentley says. “We don’t want the fee to be a hindrance.”